Generally, in a multicarrier communication scheme such as OFDM (Orthogonal Frequency Division Multiplex), transmission signals superimposed on a plurality of subcarriers are added in the time domain, so that high peak power is produced. To be more specific, when power of one subcarrier is assumed to be P, average power of a multicarrier signal including M subcarriers becomes MP, and peak power becomes M2P. More specifically, FIG. 1 illustrates a time-series power variation of the multicarrier signal when power of one subcarrier is assumed to be 1, on a complex plane. In the figure, the distance from the origin (0, 0) shows the power of the multicarrier signal and that peak power is produced in various spots.
Linearly amplifying a multicarrier signal having such high peak power requires an amplifier that can secure linearity over a wide power range, but such an amplifier causes an increase in cost and deterioration in power efficiency. Then, particularly in a multicarrier communication scheme, it is necessary to reduce peak power of a multicarrier signal beforehand and then input the result to an amplifier.
A method of reducing peak power of a multicarrier signal is disclosed in, for example, Patent Document 1.
Patent Document 1 discloses amplifying power of a multicarrier signal after converting the multicarrier signal by a function using arctangent. The arctangent is a function having mapping that gradually approximates to a predetermined value when the absolute value of an input value becomes greater, and therefore, if power of a multicarrier signal is assumed to be the input value, the mapping is always a smaller value than the predetermined value. Therefore, Patent Document 1 discloses converting power of the multicarrier signal to a smaller value than the predetermined value, reducing peak power, and then amplifying the power by an amplifier.
However, the instantaneous power of the multicarrier signal may approximate to 0 (hereinafter, this may be referred to as “zero crossing”). When the instantaneous power approximates to 0, the amplifier must keep linearity with respect to power which is close to 0, and this may also cause an increase in cost. Therefore, it is important not only to reduce peak power but also to avoid instantaneous power which is close to 0. According to Patent Document 1, arctangent is used, and, when the instantaneous power is small to a certain degree, the power is converted to a large value, so that the above-described problem can be reduced.
Moreover, as a scheme for reducing a dynamic range of a modulated signal, π/4 shift QPSK (π/4-QPSK) is well known. According to π/4-QPSK, by making the carrier phases of consecutive symbols advance by π/4 from the phase associated with transmission information, passage through the origin (zero crossing) is avoided during phase transition. By avoiding zero crossing, it is possible to reduce the dynamic range of the modulated signal. Examples of similar schemes include OQPSK (offset QPSK). Patent Document 1: Japanese Patent Application Laid-Open No. 2005-51457